1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means can comprise an array of individually controllable elements that generate the circuit pattern.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
In optical maskless scanners there is a problem known as micro overlay. This problem is related to the fact that in maskless lithography, normally two flashes of a beam are used to project the full pattern or image onto the substrate. In contrast, in mask-based lithography thirty to forty pulses are used to project the pattern or image onto the substrate causing an averaging effect, which is not present in the maskless lithography. Consequently, micro overlay occurs in mask-based lithography, but it is not a critical issue due to this averaging effect.
As mentioned above, two pulses are normally needed to project the pattern or image in maskless lithography. The first pulse or flash of the beam, e.g., laser or other appropriate light source, essentially freezes half of the image representing the pattern on the patterning means into the resist of the substrate (e.g., stamp 1). The second exposure of the same portion on the resist (e.g., stamp 2) occurs at a later time in order to get the full image into the resist.
It should be appreciated that stamps 1 and 2, which together sufficiently expose the resist to get the full image, occur separated by an arbitrary amount of time depending on the exposure routing over the whole of the wafer. Due to small vibrations in the frame system, the second pulse will not project the image on the same portion on the resist as the first pulse of the beam, implying that the image will be distorted due to absolute misplacement between the first and the second pulse. Thus, the first and the second stamps in the resist will be misplaced in relation to each other causing a distorted image on the resist.
Each pulse or flash of a beam carries a specific aerial image, allowing for projecting of the aerial image on a specific portion on the substrate. Resist will be misplaced due to the above mentioned vibrations in the frame system. Thus, the vibrations in the frame system cause a mismatch between the aerial image and the actual position of the wafer (e.g., a wafer stage).
Vibrations in the frame system in this context are small mechanical vibrations emanating from a whole mechanical system of the lithography apparatus, such as vibrations in a base frame, a metro frame, the wafer stage, lens frame structure, optical lens elements, stage positioning mechanics (short and long stroke modules), interferometric measuring devices, etc., when the lithography apparatus is up and running.
Therefore, what is needed is a system and method that reduce a positional error of an aerial image with reference to a substrate caused mainly by vibrations.